Medical implants are used in a wide variety of applications, from regulating heart rhythm (e.g. a pace maker) to improving hearing in a user or recipient. Implantable hearing prostheses, such cochlear implants, are widely used in restoring a sense of hearing to profoundly deaf persons.
In a cochlear implant, electrical stimulation signals are applied directly to the auditory nerve fibers of the recipient, thereby allowing the brain to perceive a hearing sensation that approximates the natural hearing sensation. More particularly, in operation, audio signals are received via a microphone, and are provided to a sound processing unit. The sound processing unit converts the audio signals into coded signals that are provided to a stimulator unit that uses the coded signals to generate the stimulation signals. The stimulator unit is electrically connected to an electrode array implanted in the recipient's cochlea and that delivers the stimulation signals to the recipient.
In practice, the sound processing unit is generally located externally to the recipient, and the stimulator is implanted within the recipient, usually near the mastoid and underneath the surrounding tissue. The sound processing unit and stimulator unit communicate using various wireless transmission systems, including a radio frequency (RF) link.
There is always some risk of malfunction of cochlear implants and other medical implants. Although these implants are designed to have minimal impact on a recipient's safety should they fail, the outcome of some failure modes is difficult to either control or predict. For example, failure of a random semiconductor component within an implanted device may cause localized unsafe heating of adjacent body tissue and potential recipient discomfort. Additionally, a strong impact to a recipient's head has the potential to damage the hermetic housing of an implant, thereby allowing the ingress of body fluid and egress of potentially harmful chemicals. Furthermore, upon bridging of electrically powered circuitry, the ion rich, aqueous body fluids would be subject to electric current flow, electrolysis and subsequent production of toxic substances. Under the pressure created by the electrolytic evolution of gaseous components, these toxic substances might be expelled into surrounding body tissue with dire effect to the recipient.